3d integrated circuit device fabrication with precisely controllable substrate removal

ABSTRACT

A method is provided for fabricating a 3D integrated circuit structure. According to the method, a first active circuitry layer wafer is provided. The first active circuitry layer wafer comprises a P+ portion covered by a P− layer, and the P− layer includes active circuitry. The first active circuitry layer wafer is bonded face down to an interface wafer that includes a first wiring layer, and then the P+ portion of the first active circuitry layer wafer is selectively removed with respect to the P− layer of the first active circuitry layer wafer. Next, a wiring layer is fabricated on the backside of the P− layer. Also provided are a tangible computer readable medium encoded with a program for fabricating a 3D integrated circuit structure, and a 3D integrated circuit structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to application “3D Integrated Circuit DeviceFabrication Using Interface Wafer As Permanent Carrier,” Ser. No.______, now ______, and application “3D Integrated Circuit Device HavingLower-Cost Active Circuitry Layers Stacked Before Higher-Cost ActiveCircuitry Layer,” Ser. No. ______, now ______, which were filed on thesame day as the present application and commonly assigned therewith toInternational Business Machines Corporation. These related applicationsare incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention generally relates to the field of integratedcircuits, and more particularly relates to the fabrication ofthree-dimensional (3D) integrated circuit devices.

BACKGROUND OF THE INVENTION

Three-dimensional (3D) integrated circuit devices are experiencingextremely active development in the industry. One problem experienced inthe fabrication of 3D integrated circuit devices is that general purposesubstrate thinning techniques do not allow the final substrate to beproduced with a controlled thickness that is thin enough to allowhigh-density through-silicon vias with reasonable aspect ratios to berealized. One known technique for overcoming this problem is utilizing aburied oxide layer (BOX) as the etch stop. However, this technique onlyworks for silicon-on-insulator (SOI) wafers. Further, even with an SOIwafer, this technique does not work for SOI circuits having structuresthat extend below the buried oxide, such as an embedded DRAM (e-DRAM)trench.

Another known technique for overcoming this problem is utilizing adouble buried oxide layer (double-BOX) structure. However, thistechnique greatly increases the manufacturing cost. Further, like thesingle buried oxide layer structure solution, the double-BOX techniquerequires protection of the substrate from the other wafer. Suchprotection is required because, while the SOI wafer acts as an etchstop, it does not provide selectivity between the different substrates.

Yet another known technique for overcoming this problem is to not use anetch stop but to perform “blind” thinning. However, this technique doesnot allow the wafers to be thinned aggressively and creates uniformityproblems. Further, for integrated circuits that require a high densityof 3D vias, this technique also forces the use of high aspect ratio viasthat cannot be filled with copper. Instead, tungsten has to be used forthe vias, which has three times higher resistivity than copper.

Another problem experienced in the fabrication of 3D integrated circuitdevices is that stacking three or more layers to create a multi-layerstack leads to yield loss. One technique that attempts to overcome thisproblem is to stack layers through bonding to temporary handle wafers.However, the use of such a temporary handle wafer (e.g., a glass wafer)induces overlay distortions that degrade the alignment overlay betweenthe wafers. That is, this technique does not allow high-precisionoptical alignment in subsequent lithographic steps. Withouthigh-precision optical alignment, the via density is degraded and largecapture pads with high parasitic capacitances must be used. Further, theuse of such bonding to temporary handle wafers does allow flexibility inthe way the wafers are stacked.

Another technique that attempts to overcome this problem is to simplyuse a direct face-to-face joining of the wafers. However, such directface-to-face joining is problematic because the bottom wafer (whichusually is a logic wafer) must then be used as the handle waferthroughout the stacking process. While this may be acceptable in thefabrication of a two layer stack, for a multi-layer (i.e., three or morelayer) stack this means that the logic wafer must go through manybonding and thinning steps. This increases the probability ofcatastrophic failure and loss of the entire integrated circuit,including the logic wafer that is often the most expensive wafer in thestack.

SUMMARY OF THE INVENTION

One embodiment of the present invention provides a method forfabricating a 3D integrated circuit structure. According to the method,a first active circuitry layer wafer is provided. The first activecircuitry layer wafer comprises a P+ portion covered by a P− layer, andthe P− layer includes active circuitry. The first active circuitry layerwafer is bonded face down to an interface wafer that includes a firstwiring layer, and then the P+ portion of the first active circuitrylayer wafer is selectively removed with respect to the P− layer of thefirst active circuitry layer wafer. Next, a wiring layer is fabricatedon the backside of the P− layer.

Another embodiment of the present invention provides a 3D integratedcircuit structure that includes an interface wafer, a first activecircuitry layer wafer comprising a P− layer that includes activecircuitry, a second wiring layer on the backside of the P− layer, and asecond active circuitry layer wafer comprising a P+ portion covered by aP− layer. The interface wafer includes a first wiring layer, and thefirst active circuitry layer wafer is bonded face down to the interfacewafer. The P− layer of the second active circuitry layer wafer includesactive circuitry, and the second active circuitry layer wafer is bondedface down to the second wiring layer.

Other objects, features, and advantages of the present invention willbecome apparent from the following detailed description. It should beunderstood, however, that the detailed description and specificexamples, while indicating preferred embodiments of the presentinvention, are given by way of illustration only and variousmodifications may naturally be performed without deviating from thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 11 are cross-sectional views of a process for fabricating athree-dimensional integrated circuit device according to one embodimentof the present invention;

FIG. 12 shows a 3D integrated circuit structure having a logiclayer-active circuitry layers-interface layer stack structure accordingto one embodiment of the present invention;

FIG. 13 shows a 3D integrated circuit structure in which all of thelayers are SOI wafers in accordance with one embodiment of the presentinvention;

FIG. 14 shows a 3D integrated circuit structure having circuit elementsin the interface layer of the stack structure according to oneembodiment of the present invention; and

FIG. 15 is a flow diagram of a design process used in semiconductordesign, manufacture, and/or test.

DETAILED DESCRIPTION

Preferred embodiments of the present invention will be described indetail hereinbelow with reference to the attached drawings.

Embodiments of the present invention allow precisely controllablesubstrate removal during three-dimensional (3D) integrated circuitdevice fabrication by utilizing a P+/P− substrate. The P+/P− substrateis a P+ wafer that has a P− top active layer. In one exemplaryembodiment, the P− top active layer is grown epitaxially on a P+ waferand has a thickness of between about 5 and 20 microns. The P+ wafer andP− top active layer of the P+/P− substrate allow controllable substrateremoval down to very thin layers. Thus, the present invention overcomesthe problem of a lack of controllable substrate thinning that is presentin the known and previously proposed substrate thinning techniques for3D integrated circuits. The controllability of substrate removal in thepresent invention allows low aspect ratio vias that can be filled withcopper, so as to present low resistivity. Additionally, in embodimentsof the present invention, a general purpose etch stop layer is providedthat is cost effective and robust.

FIGS. 1 to 11 illustrate a process for fabricating a three-dimensionalintegrated circuit device according to one embodiment of the presentinvention. As shown in FIG. 1, the process starts with an interfacewafer 100, which will be the interface between the stacked activecircuitry layers and a package in the completed integrated circuit. Inparticular, the exposed surface of the interface wafer 100 will carrythe C4 (controlled collapse chip connection) solder bumps in thecompleted integrated circuit. These C4 (or flip-chip) solder bumps areused to attach the integrated circuit to the package (e.g., a resin orceramic module). The interface wafer 100 is a substrate made of amaterial that is not soluble in the etchants that are utilized in thesubsequent substrate removal steps (i.e., a material that is not solublein etchants that selectively etch P+ layers with respect to P− layers).

In this embodiment, the interface wafer is not formed from a P+substrate, so it is impervious to the etching that removes thesubstrates of the wafers of the active circuitry layers of the stack.The interface wafer 100 is a silicon substrate that has through-siliconvias 102 at the same pitch as the package. Further, in this embodiment,the through-silicon vias are filled with tungsten metal. In furtherembodiments, the through-silicon vias are filled with othermetallurgies, such as copper. The through-silicon vias of the interfacewafer 100 do not need to be made of the same material as thethrough-silicon vias in the other layers of the stack. The interfacewafer 100 also has a wiring layer 104 that distributes signals and powerto the stacked layers of the integrated circuit. In this embodiment, theinterface wafer 100 is transparent to infrared radiation.

Additionally, a first active circuitry layer wafer 200 is provided. Thefirst active circuitry layer wafer 200 is formed with a P+/P− siliconsubstrate, which is a P+ wafer 202 that has a P− top active layer 204.In this embodiment, the P− top active layer 204 is grown epitaxially ona P+ wafer and has a thickness of between about 5 and 20 microns.Further, in this embodiment the P+ wafer is a boron-doped wafer with adoping concentration in the range of about 1×10¹⁸ cm−³ to 3×10²⁰ cm−³,and the P− epitaxial layer has a doping concentration of less than about1×10¹⁸ cm−³. In further embodiments, the P− epitaxial layer isnot-intentionally-doped, or doped N-type with a concentration of lessthan about 1×10¹⁸ cm−³.

Through-silicon vias 206 are etched into the P− top active circuitrylayer 204 so as to end near the P+ wafer 202. In further embodiments,the vias 206 pass through the surface of the P+ wafer 202. Activecircuitry (i.e., active components such as transistors) and one or morewiring levels 208 are formed at the top surface of the first activecircuitry layer wafer 200.

Next, as shown in FIG. 2, the first active circuitry layer wafer 200 isaligned face down to the interface wafer 100. This face-to-facealignment using two silicon wafers allows for higher-precision alignmentcompared to the case where one of the wafers utilizes a temporary handlewafer (e.g., of glass). The first active circuitry layer wafer 200 isbonded to the interface wafer 100. In this embodiment, copper-copper ora combination of copper-copper and adhesive bonding (e.g., using apolymer adhesive) is utilized. In further embodiments, othermetallurgies (such as a copper alloy or a nickel-gold alloy) areutilized.

The P+ layer 202 of the first active circuitry layer wafer 200 is thenselectively removed, as shown in FIG. 3. In this embodiment, a series ofnon-selective substrate thinning processes are first utilized (e.g.,wafer grinding and polishing), and then wet chemical etching is utilizedto remove the remaining P+ layer 202 selectively with respect to the P−layer 204. The final removal of the P+ layer 202 of the first activecircuitry layer wafer 200 is performed using a selective etchant, suchas HNA (hydrofluoric acid/nitric acid/acetic acid). Because the bulk ofthe interface wafer 100 is not soluble in this selective etchant, theprocess is very robust. Additionally, in this embodiment the interfacewafer 100 is made from a lightly doped N- or P− silicon in order toallow infrared (IR) alignment to be performed. In other embodiments, theinterface wafer 100 is also a P+ silicon substrate.

This selective removal of the P+ layer does not substantially affect theP− epitaxial layer 204, the active circuitry and wiring levels 104 and208, or the interface wafer 100 that remain. Thus, the use of the P+/P−substrate allows the P+ layer to be selectively removed, so that thewafer is controllably thinned to the thickness of the P− layer, whichcan be made very thin (e.g., about 5-20 microns thick).

Next, in this embodiment, an etch back (e.g., using reactive ionetching) is performed in order to expose the top portions of the vias206 in the P− layer 204. In other embodiments in which the vias 206 passinto the P+ layer 202, such an etch is not needed as the top portions ofthe vias are already exposed after the selective removal of the P+layer. A wiring layer 210 having insulation and one or more back end ofline (BEOL) metallization layers coupled to the vias 206 is thenpatterned onto the backside of the P− layer 204, as shown in FIG. 4. Inthis embodiment, each metallization layer of the wiring layer 210 isformed by depositing a dielectric layer, etching the dielectric layer,and depositing metal in the etched areas.

These steps are then repeated any number of times to create amulti-layer stack on the interface wafer 100. For example, in theillustrated embodiment these steps are repeated once more to create asecond active circuitry layer. More specifically, a second activecircuitry layer wafer 300 is provided, as shown in FIG. 5. The secondactive circuitry layer wafer 300 is also formed with a P+/P− siliconsubstrate, which is a P+ wafer 302 that has a P− top active circuitrylayer 304. In this embodiment, the P− top active circuitry layer 304 isgrown epitaxially and has a thickness of between about 5 and 20 microns.Through-silicon vias 306 are etched into the P− top active circuitrylayer 304 so as to end near the P+ wafer 302, and active circuitry andone or more wiring levels 308 are formed at the top surface of thesecond active circuitry layer wafer 300.

Next, as shown in FIG. 6, the second active circuitry layer wafer 300 isaligned face down to the wiring layer 210 on the first P− layer 204attached to the interface wafer 100. This face-to-face alignment usingtwo silicon wafers allows for higher-precision alignment compared to thecase where one of the wafers utilizes a temporary handle wafer (e.g., ofglass). The second active circuitry layer wafer 300 is bonded to thewiring layer 210 using copper-copper or a combination of copper-copperand adhesive bonding. In further embodiments, other metallurgies (suchas a copper alloy or a nickel-gold alloy) are utilized.

The P+ layer 302 of the second active circuitry layer wafer 300 is thenselectively removed, as shown in FIG. 7. In this embodiment, a series ofnon-selective substrate thinning processes are first utilized (e.g.,wafer grinding and polishing), and then a wet chemical etching isutilized to remove the remaining P+ layer 302 of the second activecircuitry layer wafer 300 selectively with respect to the P− layer 304of the second active circuitry layer wafer 300. This selective removalof the P+ layer 302 of the second active circuitry layer wafer 300 doesnot substantially affect the P− layers 204 and 304, the active circuitryand wiring levels 104, 208, 210, and 308, or the interface wafer 100that remain. Thus, the interface wafer 100 is robust against multiplesubstrate removal etches for removing the P+ layers of all of the activecircuitry layer wafers that are used to create the multi-layer stack.

An etch back is then performed in order to expose the top portions ofthe vias 306 in the second P− layer 304. A wiring layer 310 havinginsulation and one or more BEOL metallization layers coupled to the vias306 is then patterned onto the backside of the second P− layer 304, asshown in FIG. 8.

After the desired number of active circuitry layers are bonded to theinterface layer in this manner, the resulting structure is attached to abase wafer 800, as shown in FIG. 9. The base wafer 800 of thisembodiment is made of bulk silicon or SOI and is topped by a wiringlayer 808 having insulation and one or more BEOL metallization layers.In this embodiment, the base wafer 800 does not have through-siliconvias. In some embodiments, the base wafer includes active circuitry(e.g., transistors) and/or passive circuit elements (e.g., resistors andcapacitors). The interface layer 100 and the stack of active circuitrylayers attached to it are aligned face down to the base wafer. Thus,because they have been “flipped” twice in this embodiment, all of theactive circuitry layers that were previously stacked on the interfacewafer 100 are now face up with respect to the base wafer 800, as shownby the arrows in FIG. 9.

The top wiring layer 310 of the multi-layer stack is then bonded to thebase wafer 800. In this embodiment, copper-copper or a combination ofcopper-copper and adhesive bonding (e.g., using a polymer adhesive) isutilized. In further embodiments, other metallurgies (such as a copperalloy or a nickel-gold alloy) are utilized. The interface wafer 100 isthen thinned. In this embodiment, this thinning is achieved in twosteps. First, a combination of wafer grinding and polishing areperformed to thin the interface wafer to above the vias 102 in theinterface wafer 100. Then, the interface wafer 100 is further thinnedthrough a dry etch (e.g., using reactive ion etching) so as to form aninterface layer 101 that exposes the top portions of the vias 102, asshown in FIG. 10. A backside dielectric layer 820 is then deposited ontothe backside of the interface layer 101.

As shown in FIG. 11, the backside dielectric layer 820 is then polishedand/or etched, and contact metallizations 822 are deposited onto thevias 102 of the interface layer 101. In this embodiment, a simpleball-limiting metallization is deposited on the vias so as to allowdeposition of C4 solder bumps. In other embodiments, more complexdielectric and metallization layers are formed. The C4 solder bumps 825are then deposited onto the contact metallizations 822 to complete the3D integrated circuit structure. In this embodiment, the C4 solder bumpsare on the order of 100 μm in diameter and 200 μm or less in pitch.These C4 (or flip-chip) solder bumps are then used to bond theintegrated circuit to the package (e.g., a resin or ceramic module).

The exemplary process described above is only meant to illustrate theprinciples of the present invention. By simply varying the number,types, and order of layers that are stacked on the interface wafer, manydifferent 3D integrated circuit structures can be produced. For example,while the exemplary process described above produces a structure havinga 1+2+1 stack (1 base wafer, 2 active circuitry layers, and 1 interfacelayer), a structure having a 1+N+1 stack can be created by simplyrepeating the active circuitry layer stacking process N times asdescribed above. In the 1+N+1 stack structure, additional activecircuitry layers (each analogous to the one formed by layers 210, 204,and 208) are stacked between layer 210 of the first active circuitrylayer and layer 308 of the last (N^(th)) active circuitry layer in thestructure of FIG. 11 (see, for example, FIG. 12).

Similarly, a structure having a 1+1+1 stack can be created by onlyperforming the active circuitry layer stacking process one time. In the1+1+1 stack structure, the second active circuitry layer (the one formedby layers 310, 304, and 308) is absent from the structure of FIG. 11.Thus, the process of the present invention for stacking multiple activecircuitry layers on top of a base wafer can also be used in a consistentmanner to stack only one active circuitry layer on a base wafer.

In further embodiments, a logic wafer (i.e., a wafer with logiccircuitry) is used as the base wafer (i.e., the base wafer includeslogic circuitry). For example, FIG. 12 shows a 3D integrated circuitstructure having a logic wafer-active circuitry layers-interface layerstack structure according to one embodiment of the present invention.This exemplary embodiment has an interface layer that is stacked on topof N active circuitry layers, which are memory layers, that are in turnstacked on top of a logic wafer. The memory layers are any type ofmemory, such as SRAM memory, e-DRAM memory, or a combination of the two.The logic wafer contains control and/or logic circuitry, such as amemory controller or a processor core. In another embodiment, theinterface layer and only one active circuitry layer (e.g., memory layer)are stacked on top of the logic wafer.

Additionally, one or more of the wafers used in the process describedabove can be a silicon-on-insulator (SOI) wafer. For example, FIG. 13shows a 3D integrated circuit structure in which all of the layers areformed on SOI wafers in accordance with one embodiment of the presentinvention. In an alternative embodiment, the base wafer is an SOIsubstrate (as in FIG. 13), while the wafers for the active circuitrylayers and the interface layer are bulk silicon wafers (as in FIG. 12).In yet another embodiment, the base wafer is an SOI substrate (as inFIG. 13), the wafers for the active circuitry layers include both an SOIwafer (as in FIG. 13) and a bulk silicon wafer (as in FIG. 12), and thewafer for the interface layer is either an SOI or bulk silicon wafer.

FIG. 14 shows a 3D integrated circuit structure having circuit elementsin the interface layer of the stack structure according to oneembodiment of the present invention. In this embodiment, the interfacelayer includes additional functionality through the provision of activecircuitry and/or passive circuit elements. For example, the interfacelayer can include a decoupling capacitor layer in order to stabilize thevoltage grid. Alternatively or additionally, the interface layer caninclude voltage regulation circuitry formed by active transistors aswell as passive elements such as decoupling capacitors.

Accordingly, embodiments of the present invention utilize a P+/P−substrate to fabricate a 3D integrated circuit device. This allowsprecisely controllable substrate removal down to very thin layers. Thus,the present invention overcomes the problem of a lack of controllablesubstrate thinning that is present in the known substrate thinningtechniques for 3D integration. This allows low aspect ratio vias thatcan be filled with copper, so as to present low resistivity.Additionally, in embodiments of the present invention, a general purposeetch stop layer is provided that is cost effective and robust.

The embodiments of the present invention described above are meant to beillustrative of the principles of the present invention. These devicefabrication processes are compatible with conventional semiconductorfabrication methodology, and thus various modifications and adaptationscan be made by one of ordinary skill in the art. All such modificationsstill fall within the scope of the present invention. For example, thevarious layer thicknesses, material types, deposition techniques, andthe like discussed above are not meant to be limiting.

Furthermore, some of the features of the examples of the presentinvention may be used to advantage without the corresponding use ofother features. As such, the foregoing description should be consideredas merely illustrative of the principles, teachings, examples andexemplary embodiments of the present invention, and not in limitationthereof.

It should be understood that these embodiments are only examples of themany advantageous uses of the innovative teachings herein. In general,statements made in the specification of the present application do notnecessarily limit any of the various claimed inventions. Moreover, somestatements may apply to some inventive features but not to others. Ingeneral, unless otherwise indicated, singular elements may be in theplural and vice versa with no loss of generality.

The circuit as described above is part of the design for an integratedcircuit chip. The chip design is created in a graphical computerprogramming language, and stored in a computer storage medium (such as adisk, tape, physical hard drive, or virtual hard drive such as in astorage access network). If the designer does not fabricate chips or thephotolithographic masks used to fabricate chips, the designer transmitsthe resulting design by physical means (e.g., by providing a copy of thestorage medium storing the design) or electronically (e.g., through theInternet) to such entities, directly or indirectly. The stored design isthen converted into the appropriate format (e.g., GDSII) for thefabrication of photolithographic masks, which typically include multiplecopies of the chip design in question that are to be formed on a wafer.The photolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

The method as described above is used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare chip, or in a packaged form. Inthe latter case, the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case, the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboard,or other input device, and a central processor.

FIG. 15 shows a block diagram of an exemplary design flow 900 used forexample, in semiconductor IC logic design, simulation, test, layout, andmanufacture. Design flow 900 includes processes and mechanisms forprocessing design structures or devices to generate logically orotherwise functionally equivalent representations of the designstructures and/or devices described above and shown in FIGS. 1-14. Thedesign structures processed and/or generated by design flow 900 may beencoded on machine-readable transmission or storage media to includedata and/or instructions that when executed or otherwise processed on adata processing system generate a logically, structurally, mechanically,or otherwise functionally equivalent representation of hardwarecomponents, circuits, devices, or systems. Design flow 900 may varydepending on the type of representation being designed. For example, adesign flow 900 for building an application specific IC (ASIC) maydiffer from a design flow 900 for designing a standard component or froma design flow 900 for instantiating the design into a programmablearray, for example a programmable gate array (PGA) or a fieldprogrammable gate array (FPGA) offered by Altera® Inc. or Xilinx® Inc.

FIG. 15 illustrates multiple such design structures including an inputdesign structure 920 that is preferably processed by a design process910. Design structure 920 may be a logical simulation design structuregenerated and processed by design process 910 to produce a logicallyequivalent functional representation of a hardware device. Designstructure 920 may also or alternatively comprise data and/or programinstructions that when processed by design process 910, generate afunctional representation of the physical structure of a hardwaredevice. Whether representing functional and/or structural designfeatures, design structure 920 may be generated using electroniccomputer-aided design (ECAD) such as implemented by a coredeveloper/designer. When encoded on a machine-readable datatransmission, gate array, or storage medium, design structure 920 may beaccessed and processed by one or more hardware and/or software moduleswithin design process 910 to simulate or otherwise functionallyrepresent an electronic component, circuit, electronic or logic module,apparatus, device, or system such as those shown in FIGS. 1-14. As such,design structure 920 may comprise files or other data structuresincluding human and/or machine-readable source code, compiledstructures, and computer-executable code structures that when processedby a design or simulation data processing system, functionally simulateor otherwise represent circuits or other levels of hardware logicdesign. Such data structures may include hardware-description language(HDL) design entities or other data structures conforming to and/orcompatible with lower-level HDL design languages such as Verilog andVHDL, and/or higher level design languages such as C or C++.

Design process 910 preferably employs and incorporates hardware and/orsoftware modules for synthesizing, translating, or otherwise processinga design/simulation functional equivalent of the components, circuits,devices, or logic structures shown in FIGS. 1-14 to generate a netlist980 which may contain design structures such as design structure 920.Netlist 980 may comprise, for example, compiled or otherwise processeddata structures representing a list of wires, discrete components, logicgates, control circuits, I/O devices, models, etc. that describes theconnections to other elements and circuits in an integrated circuitdesign. Netlist 980 may be synthesized using an iterative process inwhich netlist 980 is resynthesized one or more times depending on designspecifications and parameters for the device. As with other designstructure types described herein, netlist 980 may be recorded on amachine-readable data storage medium or programmed into a programmablegate array. The medium may be a non-volatile storage medium such as amagnetic or optical disk drive, a programmable gate array, a compactflash, or other flash memory. Additionally, or in the alternative, themedium may be a system or cache memory, buffer space, or electrically oroptically conductive devices and materials on which data packets may betransmitted and intermediately stored via the Internet, or othernetworking suitable means.

Design process 910 may include hardware and software modules forprocessing a variety of input data structure types including netlist980. Such data structure types may reside, for example, within libraryelements 930 and include a set of commonly used elements, circuits, anddevices, including models, layouts, and symbolic representations, for agiven manufacturing technology (e.g., different technology nodes, 32 nm,45 nm, 90 nm, etc.). The data structure types may further include designspecifications 940, characterization data 950, verification data 960,design rules 970, and test data files 985 which may include input testpatterns, output test results, and other testing information. Designprocess 910 may further include, for example, standard mechanical designprocesses such as stress analysis, thermal analysis, mechanical eventsimulation, process simulation for operations such as casting, molding,and die press forming, etc. One of ordinary skill in the art ofmechanical design can appreciate the extent of possible mechanicaldesign tools and applications used in design process 910 withoutdeviating from the scope and spirit of the invention. Design process 910may also include modules for performing standard circuit designprocesses such as timing analysis, verification, design rule checking,place and route operations, etc.

Design process 910 employs and incorporates logic and physical designtools such as HDL compilers and simulation model build tools to processdesign structure 920 together with some or all of the depictedsupporting data structures along with any additional mechanical designor data (if applicable), to generate a second design structure 990.Design structure 990 resides on a storage medium or programmable gatearray in a data format used for the exchange of data of mechanicaldevices and structures (e.g., information stored in a IGES, DXF,Parasolid XT, JT, DRG, or any other suitable format for storing orrendering such mechanical design structures). Similar to designstructure 920, design structure 990 preferably comprises one or morefiles, data structures, or other computer-encoded data or instructionsthat reside on transmission or data storage media and that whenprocessed by an ECAD system generate a logically or otherwisefunctionally equivalent form of one or more of the embodiments of theinvention shown in FIGS. 1-14. In one embodiment, design structure 990may comprise a compiled, executable HDL simulation model thatfunctionally simulates the devices shown in FIGS. 1-14.

Design structure 990 may also employ a data format used for the exchangeof layout data of integrated circuits and/or symbolic data format (e.g.,information stored in a GDSII (GDS2), GL1, OASIS, map files, or anyother suitable format for storing such design data structures). Designstructure 990 may comprise information such as, for example, symbolicdata, map files, test data files, design content files, manufacturingdata, layout parameters, wires, levels of metal, vias, shapes, data forrouting through the manufacturing line, and any other data required by amanufacturer or other designer/developer to produce a device orstructure as described above and shown in FIGS. 1-14. Design structure990 may then proceed to a stage 995 where, for example, design structure990: proceeds to tape-out, is released to manufacturing, is released toa mask house, is sent to another design house, is sent back to thecustomer, etc.

1. A method for fabricating a 3D integrated circuit, the methodcomprising the steps of: providing an interface wafer, the interfacewafer including a first wiring layer and through-silicon vias; providinga first active circuitry layer wafer comprising a P+ portion covered bya P− layer, the P− layer of the first active circuitry layer waferincluding active circuitry and through-silicon vias; bonding the firstactive circuitry layer wafer face down to the interface wafer; afterbonding the first active circuitry layer wafer face down to theinterface wafer, selectively removing the P+ portion of the first activecircuitry layer wafer with respect to the P− layer of the first activecircuitry layer wafer; after selectively removing the P+ portion of thefirst active circuitry layer wafer, fabricating a second wiring layer onthe backside of the P− layer; providing a second active circuitry layerwafer comprising a P+ portion covered by a P− layer, the P− layer of thesecond active circuitry layer wafer including active circuitry; bondingthe second active circuitry layer wafer face down to the second wiringlayer; after bonding the second active circuitry layer wafer face downto the second wiring layer, selectively removing the P+ portion of thesecond active circuitry layer wafer with respect to the P− layer of thesecond active circuitry layer wafer; after selectively removing the P+portion of the second active circuitry layer wafer, fabricating a thirdwiring layer on the backside of the P− layer of the second activecircuitry layer wafer; providing a base wafer, the base wafer includinga fourth wiring layer; bonding the third wiring layer face down to thebase wafer; and after bonding the third wiring layer face down to thebase wafer, thinning the interface wafer so as to form an interfacelayer, and forming metallizations comprising solder bumps on theinterface layer, the solder bumps being coupled through thethrough-silicon vias in the interface layer to the first wiring layer,wherein the interface wafer is formed of a material that is not solublein an etchant used in the selectively removing step to selectively etchthe P+ portion of the first active circuitry layer wafer with respect tothe P− layer of the first active circuitry layer wafer.
 2. A method forfabricating a 3D integrated circuit structure, the method comprising thesteps of: providing a first active circuitry layer wafer comprising a P+portion covered by a P− layer, the P− layer of the first activecircuitry layer wafer including active circuitry; bonding the firstactive circuitry layer wafer face down to an interface wafer thatincludes a first wiring layer; after bonding the first active circuitrylayer wafer face down to the interface wafer, selectively removing theP+ portion of the first active circuitry layer wafer with respect to theP− layer of the first active circuitry layer wafer; and afterselectively removing the P+ portion of the first active circuitry layerwafer, fabricating a wiring layer on the backside of the P− layer. 3.The method of claim 2, further comprising the steps of: providing a basewafer, the base wafer including a second wiring layer; and bonding thewiring layer on the backside of the P− layer face down to the basewafer.
 4. The method of claim 3, further comprising the step of: afterbonding the wiring layer on the backside of the P− layer face down tothe base wafer, thinning the interface wafer so as to form an interfacelayer, and forming metallizations on the interface layer, themetallizations being coupled through vias in the interface layer to thefirst wiring layer.
 5. The method of claim 3, wherein the base waferincludes logic circuitry.
 6. The method of claim 2, wherein theinterface wafer is formed of a material that is not soluble in anetchant used in the selectively removing step to selectively etch the P+portion of the first active circuitry layer wafer with respect to the P−layer of the first active circuitry layer wafer.
 7. The method of claim2, wherein the first active circuitry layer wafer further comprisesthrough-silicon vias.
 8. The method of claim 2, further comprising thesteps of: providing another active circuitry layer wafer comprising a P+portion covered by a P− layer, the P− layer of the other activecircuitry layer wafer including active circuitry; bonding the otheractive circuitry layer wafer face down to the wiring layer on thebackside of the P− layer of the previous active circuitry layer wafer;after bonding the other active circuitry layer wafer face down,selectively removing the P+ portion of the other active circuitry layerwafer with respect to the P− layer of the other active circuitry layerwafer; and after selectively removing the P+ portion of the other activecircuitry layer wafer, fabricating another wiring layer on the backsideof the P− layer of the other active circuitry layer wafer.
 9. The methodof claim 8, further comprising the step of: repeating N times the stepsof providing another active circuitry layer wafer, bonding the otheractive circuitry layer wafer, selectively removing the P+ portion of theother active circuitry layer wafer, and fabricating another wiring layeron the backside of the P− layer of the other active circuitry layerwafer.
 10. The method of claim 9, further comprising the steps of:providing a base wafer, the base wafer including a second wiring layer;and bonding the wiring layer on the backside of the P− layer of the Nthwafer face down to the base wafer.
 11. A tangible computer readablemedium encoded with a program for fabricating a 3D integrated circuitstructure, the program comprising instructions for performing the stepsof: providing a first active circuitry layer wafer comprising a P+portion covered by a P− layer, the P− layer of the first activecircuitry layer wafer including active circuitry; bonding the firstactive circuitry layer wafer face down to an interface wafer thatincludes a first wiring layer; after bonding the first active circuitrylayer wafer face down to the interface wafer, selectively removing theP+ portion of the first active circuitry layer wafer with respect to theP− layer of the first active circuitry layer wafer; and afterselectively removing the P+ portion of the first active circuitry layerwafer, fabricating a wiring layer on the backside of the P− layer. 12.The tangible computer readable medium of claim 11, wherein the programfurther comprises instructions for performing the steps of: providing abase wafer, the base wafer including a second wiring layer; and bondingthe wiring layer on the backside of the P− layer face down to the basewafer.
 13. The tangible computer readable medium of claim 12, whereinthe program further comprises instructions for performing the step of:after bonding the wiring layer on the backside of the P− layer face downto the base wafer, thinning the interface wafer so as to form aninterface layer, and forming metallizations on the interface layer, themetallizations being coupled through vias in the interface layer to thefirst wiring layer.
 14. The tangible computer readable medium of claim11, wherein the interface wafer is formed of a material that is notsoluble in an etchant used in the selectively removing step toselectively etch the P+ portion of the first active circuitry layerwafer with respect to the P− layer of the first active circuitry layerwafer.
 15. The tangible computer readable medium of claim 11, whereinthe program further comprises instructions for performing the steps of:providing another active circuitry layer wafer comprising a P+ portioncovered by a P− layer, the P− layer of the other active circuitry layerwafer including active circuitry; bonding the other active circuitrylayer wafer face down to the wiring layer on the backside of the P−layer of the previous active circuitry layer wafer; after bonding theother active circuitry layer wafer face down, selectively removing theP+ portion of the other active circuitry layer wafer with respect to theP− layer of the other active circuitry layer wafer; and afterselectively removing the P+ portion of the other active circuitry layerwafer, fabricating another wiring layer on the backside of the P− layerof the other active circuitry layer wafer.
 16. The tangible computerreadable medium of claim 15, wherein the program further comprisesinstructions for performing the step of: repeating N times the steps ofproviding another active circuitry layer wafer, bonding the other activecircuitry layer wafer, selectively removing the P+ portion of the otheractive circuitry layer wafer, and fabricating another wiring layer onthe backside of the P− layer of the other active circuitry layer wafer.17. The tangible computer readable medium of claim 16, wherein theprogram further comprises instructions for performing the steps of:providing a base wafer, the base wafer including a second wiring layer;and bonding the wiring layer on the backside of the P− layer of the Nthwafer face down to the base wafer.
 18. A 3D integrated circuit structurecomprising: an interface wafer, the interface wafer including a firstwiring layer; a first active circuitry layer wafer comprising a P− layerthat includes active circuitry, the first active circuitry layer waferbeing bonded face down to the interface wafer; a second wiring layer onthe backside of the P− layer; and a second active circuitry layer wafercomprising a P+ portion covered by a P− layer, the P− layer of thesecond active circuitry layer wafer including active circuitry, thesecond active circuitry layer wafer being bonded face down to the secondwiring layer.
 19. The 3D integrated circuit structure of claim 18,wherein the interface wafer is formed of a material that is not solublein an etchant that can selectively remove the P+portion of the secondactive circuitry layer wafer with respect to the P− layer of the secondactive circuitry layer wafer.
 20. The 3D integrated circuit structure ofclaim 18, wherein the P− layer of the first active circuitry layer waferfurther includes through-silicon vias.
 21. The 3D integrated circuitstructure of claim 18, wherein the first active circuitry layer wafercomprises a bulk silicon wafer and the second active circuitry layerwafer comprises a bulk silicon wafer.
 22. The 3D integrated circuitstructure of claim 18, wherein the first active circuitry layer wafercomprises an SOI wafer and the second active circuitry layer wafercomprises a bulk silicon wafer.
 23. The 3D integrated circuit structureof claim 18, wherein the first active circuitry layer wafer comprises anSOI wafer and the second active circuitry layer wafer comprises an SOIwafer.
 24. The 3D integrated circuit structure of claim 18, wherein theinterface wafer further includes active circuitry and/or passive circuitelements.
 25. The 3D integrated circuit structure of claim 18, whereinthe interface wafer further includes decoupling capacitors and/orvoltage regulation circuitry.